1. Field of the Invention
The present invention relates to a magnetoresistance effect type thin film magnetic head (referred to as "thin film MR head" hereinafter) including a magnetoresistive element (referred to as "MR element" hereinafter) having its electrical resistance changed according to a change in an applied signal magnetic field for detecting a change in the signal magnetic field recorded on a magnetic recording medium, and more particularly, to an improvement of a structure of a thin film MR head for reducing Barkhausen noise.
2. Description of the Background Art
It is known that a thin film MR head has many advantages in comparison with a coil type bulk magnetic head. A thin film MR head responses to magnetic flux by receiving a signal magnetic field recorded on a magnetic recording medium such as a magnetic tape and changes its internal resistance according to the change in the direction of magnetization within a magnetoresistive element. Therefore, the value of signal is independent of the speed of the magnetic recording medium. This thin MR head is expected to have a great potential as a reproduction magnetic head for a fixed head type PCM (Pulse Code Modulation) recorder in which high density recording is carried out by virtue of high integration density and multi-element easily obtained by the manufacturing technique of a semiconductor.
From the fact that an MR element indicates a quadratic curve with respect to an external magnetic field, it is necessary to arrange the element configuration in stripes to make sure of stability of the MR element. In order to obtain linear response of the MR element as well, bias magnetic field to the MR element is needed.
FIG. 8 shows a perspective view of a conventional thin film MR head in the proximity of an MR element. Referring to FIG. 8, a conventional thin film MR head includes an MR element 1, a lead electrode 2 connected to the ends of the MR element 1, and a bias electrode 3 located beneath the MR element 1 for applying a bias magnetic field to the MR element 1. In this thin film MR head, the magnetic field exerted from the end of a gap (arrow A) is applied to the MR element 1, whereby the MR element 1 is magnetized.
The reproduction output of the MR element reflects the magnetization of the MR element 1. The MR element 1 is arranged such that the input direction of a magnetic field is in the direction of the hard magnetizing axis. In the ideal case where the magnetization direction of the MR element 1 is moved in a rotation mode, My (magnetization in the direction of the y axis in FIG. 8) takes the linear function of Hy (magnetic field in the y direction), and the output of the MR element 1 changes in a quadratic functional manner with respect to the input magnetic field. FIG. 9A shows the relationship between an input magnetic field and a MR element output in an ideal case where there is no noise. The output of the MR element 1 is saturated according to the saturation of My in a high magnetic field.
Although the ideal MR element output is as described above, a change in My occurs not only in a rotation mode in an actual MR element. Magnetic domain disintegration occurs in the MR element to cause displacement of the magnetic domain. Particularly when the track width of the MR element is reduced, a change in My becomes significant by displacement of magnetic domain on account of magnetostatic energy. Displacement of magnetic domain causes a discontinuous change in My called Barkhausen jump (referred to as "B-jump" hereinafter). The relationship between the MR element output and input magnetic field is shown in FIG. 9B where there is a mixture of the abovedescribed rotation mode and displacement of magnetic domain. The B-jump results in noise in the reproduced output to degrade greatly the S/N ratio of the MR head. It is indispensable to suppress the B-jump in a MR element to obtain a superior MR head.
It is conventionally known to establish a state of single magnetic domain for the MR element by applying a weak magnetic field, of several oersted in the direction of easy magnetizing axis of the MR element in order to suppress the B-jump.
A thin film magnetic head employing such a MR element includes a yoke type shown in FIG. 10 and a shield type shown in FIG. 11. Referring to FIG. 10, a yoke type thin film magnetic head includes a lower yoke 4 forming a magnetic path, an upper yoke 5 divided into two in the front and back direction, and a MR element 1 and a bias electrode 3 arranged between the upper yoke 5 and the lower yoke 5. There is an insulating layer 6 therebetween for insulating the MR element 1 and the bias electrode 3. Here, the "front and back direction" denotes the horizontal direction in FIG. 10 where the left side is the front and the right side is the back. The magnetic flux generated from a magnetic recording medium 7 is introduced through the gap portion 8 in the front end portion of upper and lower yokes 5 and 4 into a magnetic circuit formed of the upper yoke 5 and the lower yoke 5. More specifically, the magnetic flux flows from the disconnected portion of the upper front yoke 5a to pass through the MR element 1, and then reenters the upper yoke 5 from the disconnected portion of the upper back yoke 5b. The magnetic flux passes through the lower yoke 4 to return to the magnetic recording medium 7. The lower yoke 4 also serves as the substrate of the thin film magnetic head, and is formed of a soft magnetic material such as ferrite.
A shield type thin film magnetic head shown in FIG. 11 includes a MR element 1, a lead wire 2, and a bias electrode 3 between a pair of upper and lower high permeability magnetic substances 9a and 9b with an insulating layer 10 therebetween. The shield thin film magnetic head has the magnetic field generated from the magnetic recording medium 7 directly applied to the MR element 1.
In comparing the above-described two types of thin film magnetic heads employing MR elements, it is known that the yoke type is advantageous than the shield type from the standpoint of improving resolution of a signal and the lifetime of a MR element.
For carrying out unification of magnetic domain of a MR element, the method shown in FIG. 12 where a MR element 1 is magnetically shielded, or the method of applying a weak magnetic field in one direction to the MR element are known. Regarding the method of applying a weak magnetic field to a MR element, there is one method wherein magnetic films with high coercive force are formed at the ends of a MR element and magnetized in the longitudinal direction of the MR element. These magnetic films make a weak magnetic field to the MR element because of its residual magnetization in the same direction as they are magnetized. This method is already proposed in Japanese Patent Laying-Open No. 60-59518 filed by the applicant of the present application. The MR element disclosed therein has a conductor portion 2 formed at the end portions of the MR element 1 with high coercive force films 11 thereunder as shown in FIG. 14 in contrary to the structure shown in FIG. 13 of a MR element in conventional thin film magnetic heads of FIGS. 10 and 11.
The Japanese Patent Laying-Open No. 60-59518 discloses the shield type thin film magnetic head shown in FIGS. 15A and 15B as an embodiment employing such a MR element.
The reference characters in FIGS. 15A and 15B are equivalent to those of FIGS. 11 and 14. Therefore, detailed description of the structure and operation thereof will not be repeated.
As the material of each high coercive force film 11, an electroless Co-P plated film is used by virtue of its easy formation process.
According to a single magnetic domain theory, a ferromagnetic thin film unified in magnetic domain has a "jump" generated due to magnetization switching in the magnetization curve when the angle of the easy magnetizing axis with respect to a signal magnetic field exceeds a right angle by a certain angle. Because there is anisotropic dispersion in the magnetic anisotropy of a ferromagnetic thin film such as Ni-Fe implementing a MR device, the easy magnetizing axis is inclined within a range of angle with respect to a specified direction of easy magnetizing axis. Therefore, the characteristic of the MR element has this "jump" in various positions with respect to the applied magnetic field. Because this "jump" caused by switching results in switching noise, it is necessary to move the "jump" to the side of the non-operating point of the magnetic head. A method of forming the easy magnetizing axis of a MR device inclined at a predetermined angle with respect to the longitudinal direction of the MR element is known for this purpose taking into consideration the anisotropic dispersion of the MR element. (Refer to Komoda, Minakata, Joint Meeting promoted by Kansai Branch Offices of Electricity Related Society, S37, 1988.)
A MR head having a high coercive force film 11 arranged at both ends of a MR element can be applied to not only a shield type but also to a yoke type. The structure of a yoke type thin film magnetic head having high coercive force films 11 formed at the ends of a MR element is shown in FIGS. 16A and 16B. The reference characters of FIGS. 16A and 16B correspond to those of FIGS. 10 and 14 so that the details of the structure and operation will not be repeated.
The above-described high coercive force film carries out unification of magnetic domain of a MR element to suppress Barkhausen noise by applying a weak magnetic field to a MR element. However, because the intensity of the magnetic field of the high coercive force film decreases according to the distance from the high coercive force film, there will be almost no magnetic field by the high coercive force film applied in the center of the MR element if the length of the MR element is increased in the direction of the track width. Therefore, unification of magnetic domain is carried out easily when the length of the MR element is small. However if the MR element is long, there is a possibility of the unification of magnetic domain not thoroughly carried out resulting in the generation of a magnetic domain wall. Therefore, the output curve of the MR element is as shown in FIG. 9C, resulting in a problem of Barkhausen noise generation.
In the case of a yoke type, the state of magnetic domain becomes unstable due to the increase in the yoke length in the direction of the track width, leading to a possibility of an unstable magnetization change appearing discontinuously as a change in resistance of the MR element.
If the track width is increased to obtain a great output signal, a greater magnetic field formed by the high coercive force film is required, so that the film thickness of the high coercive force film 11 must be accordingly increased. For example, an analog signal reproduction track width is approximately 600 .mu.m of a DCC (Digital Compact Cassette) for reproducing an analog signal of a compact cassette. With the structure shown in FIGS. 16A and 16B, the length of the MR element 1 is increased if the width of the yoke is great for sufficiently introducing a signal from a magnetic tape, resulting in a greater distance between the high coercive force films 11 formed at both ends of the MR element 1. The intensity of the magnetic field is inversely proportional to a square of the distance between the magnetic substances. Therefore, the volume of the high coercive force film 11 must be quadrupled to form a magnetic field of the same intensity when the distance is doubled.
In the method of manufacturing a thin film magnetic head, increasing the film thickness of the high coercive force film 11 is typical to increase the volume thereof. If the film thickness is increased, the distance between the MR element 1 and the conductor 2 will also be increased. As a result, the electrical resistance across the MR element 1 and the conductor 2 is increased due to the increase in film thickness of the high coercive force film 11 to provide an unstable output value or the problem of disconnection. Furthermore, in the case of a thin film magnetic head of multitracks, the resistance of the MR element will vary for each track to reduce the production yield. This also gave a disadvantage of difficulty in adjusting the signal processing circuit.